The invention relates to a method for producing a plastic article, comprising extruding a tubular preform from an extrudate and also forming the preform into a hollow body inside a blow molding tool having at least two halves by applying differential pressure, wherein at least one insert is introduced into the interior of the plastic article.
The invention also relates to an intermediate frame for a blow molding tool.
In particular when producing large plastic tanks by extrusion blow molding, it is often desirable to introduce inserts into the tank to be completed. For example, motor vehicle fuel tanks made of thermoplastic material are often provided with functional components/inserts. The inserts can be introduced manually through one or more inspection openings provided in the tank after said tank has been produced. In particular in the case of fuel tanks made of thermoplastic material, it is problematic that the plastics used for this purpose, generally HDPE, are not diffusion-resistant to hydrocarbons. For this reason, fuel tanks made of thermoplastic material are produced from co-extrudates with barrier layers for hydrocarbons. In this way it is possible to ensure the leak-tightness of the fuel tank to the greatest extent, even though, in the case of a multi-layer wall structure, the barrier layer is damaged/interrupted by openings that are subsequently to be provided in the tank wall, such that potential leakage points which have to be sealed in an expensive manner once the tank has been completed are created in the region of openings in the tank wall. In particular in order to introduce inserts into the tank which are larger than the inspection openings to be provided, it is possible, using the method disclosed in DE 602 18 237 T2, for the inserts to be encapsulated by extrusion blow molding during the shaping of the tank. However, the fastening of some of the inserts, for example the valves, still requires a plurality of openings in the tank wall.
Recently every effort has been directed at introducing all the inserts, as far as possible, into the fuel tank already during its production, in order to keep the number of openings required in the tank wall as low as possible.
The use of intermediate frames for blow molding has become known in the prior art. Thus, DE 102 31 86 A1 describes a method of the generic type for producing a plastic hollow body, in which the tubular plastic preform is cut into two sheet-like semifinished products and the semifinished products are inserted separately into a blow molding tool through a removable intermediate frame. Subsequently, two half-shells are formed, the tool halves being separated from one another by the removable intermediate frame, such that the half-shells do not make contact with one another. After the tool halves have been opened and the intermediate frame has been removed, inserts are attached to the inner side of at least one half-shell. Subsequently, the half-shells are joined to form the plastic hollow body.
This procedure gives rise to difficulties in the handling of the preform which is split into sheet-like semifinished products. The preform is very unstable owing to the greatly heated plastic. The insertion of the sheet-like semifinished products is additionally made more difficult by virtue of the fact that the two semifinished products must not make mutual contact either during transportation to the blow molding tool or in the blow molding tool, since they would otherwise stick to one another. The intermediate frame provides only a limited remedy here.
DE 10 2009 030 492 A1 proposes slitting open the tubular preform to give a C-shaped cross section and then inserting said preform into the blow molding tool. As a result of this C-shaped cross section, the more dimensionally stable round cross-sectional shape is largely retained. The interior of the preform is accessible via the slit, and therefore inserts can be introduced before the final blow molding. As a result of the only partial opening of the preform, however, the scope for movement for introducing in particular large inserts is limited.